Generally, a deflection yoke consists of a pair of horizontal deflection coils and a pair of vertical deflection coils which produce magnetic fields for deflecting the beams from the electron gun in the horizontal and the vertical directions, respectively.
Particularly, in color monitors and color television receivers having three electron gun type, the three beams of red, green and blue colors are not converged into a single point due to the mechanical deviations of the deflection yoke or the radius of curvature (comma aberration), thereby producing a misconvergence phenomenon.
Therefore there is used a self convergence method in which the horizontal deflection coils produce pin cushion type magnetic fields, and the vertical deflection coils produce barrel shaped magnetic fields in order to generate a distortion. In such a case, the magnetic fields are distorted, with the result that the focus becomes degraded, and that there is a limit in correcting the misconvergence phenomenon, thereby making it impossible to obtain a stable picture.
That is, as for the trilemma which shows the characteristics when matching the cathode ray tube and the deflection coils, if the size of the cathode ray tube is 16 inches, the trilemma has negative characteristics. As shown in FIG. 1, the red and blue beams are accurately converged over the central portion of the picture so that no misconvergence is produced, but the red and blue beams cross each other to produce a misconvergence along the upper and lower peripheries.
The usual deflection yolk is constituted such that: a pair of horizontal coils 7, 7' are installed within a coil separator 2 as a part of the deflection yolk 1; a pair of vertical deflection coils 6,6' wound around a pair of ferrite cores 3, 3' are fixedly installed on the outside of the tube neck; a neck portion 4 for accommodating an electron gun is formed on the top of the coil separator 2; and a pair of saturable reactors 8, 8' are assembled to the neck portion 4.
Japanese patent No. sho. 62-23695 describes in detail the conventional technique using a saturable reactor which is capable of correcting the misconvergence of a cathode ray tube.
That is, as shown in FIGS. 2 to 4, the saturable reactor 8, 8' are disposed on the opposite sides where there is no vertical deflection coils 6, 6' which is wound around the ferrite cores 3, 3' which are in turn fixed by means of clamp 5, 5'.
Further, the saturable reactors 8, 8' are constituted such that: horizontal correcting coils 18, 18' are wound on I-shaped drum cores 16a-16d; dc magnetic field biasing magnets 9, 9' installed on the drum cores 16a-16d are accommodated within a case; and vertical correcting coils 17, 17' are installed on the outside of the case.
Further, the saturable reactors 8, 8' matched in an inverse form are connected to the horizontal deflection coils 7, 7', and the polarities of the magnets 9, 9' having a flux density of 300-600 Gauss are disposed in opposite directions. Further, the magnets 9, 9' are connected to the vertical correcting coils 17, 17', and diodes D1, D2 are connected to the vertical correcting coils 17, 17' in inverse polarities.
Now description will be made for the case where an inverse trilemmas pattern produced in a cathode ray tube is corrected using the conventional convergence correcting device constituted as above. It is assumed that the inverse direction exists between the magnetic fluxes of the vertical correcting coils 17, 17' and the leakage magnetic fluxes of the horizontal correcting coils 18, 18'. Then, the magnetic fluxes acting on the saturable reactors 8, 8' deflects the electron beam up to the middle position of the vertical deflecting direction owing to the threshold values of the diodes D1, D2. Under these conditions, the opposite terminal voltages of the vertical correcting coils 17, 17' become the same as each other, and therefore, the misconvergence pattern of the lateral line can be corrected by nonlinearly varying the control magnetic fields and by varying the turn ratio between the vertical deflection coils 6, 6' and the horizontal correcting coils 17, 17'.
In this conventional convergence correcting device, diodes are used on the vertical correcting coils, and the saturable reactors are installed at the contact portions of the ferrite cores in order to utilize the leakage magnetic fluxes of the vertical coils. This brings the results that product deflects are liable to occur depending on the assembling conditions, that the landing characteristics are changed by the strong magnets having a flux density of 300-600 Gauss, and that the manufacturing cost are raised due to the requirement of special diodes suiting to the misconvergence characteristics.
Meanwhile, there is another correcting method which is constituted such that: the 4 corners of the picture are adjusted by means of vertical deflecting coils; and the misconvergences deviated in the horizontal direction from the Y axis are corrected by an additional means.
In this method, however, the peripheral portions are properly corrected, but the central portion is over-corrected, thereby making it impossible to achieve proper corrections of misconvergences.